This invention relates to an external memory interface circuit included in a signal processing circuit which is for processing an input signal by using an external memory. Such a signal processing circuit is particularly useful in a voice signal processor of a microprogram control type for processing a voice signal.
In such a voice signal processor, it is often required to generate echo sound by processing an input signal, such as a voice signal, in order to obtain good sound effects. For this purpose, the voice signal processor comprises a signal processing circuit for processing the input signal and an external memory having a large capacity. The external memory is for giving a predetermined delay to the input signal. As well known in the art, the input signal having the predetermined delay is used for generating the echo sound.
The signal processing circuit is connected to an external central processing unit (CPU) and carries out a signal processing operation in accordance with a predetermined signal processing program. The predetermined signal processing program is transferred from the external central processing unit to the signal processing circuit on starting of the signal processing operation. In order to temporarily storing the predetermined signal processing program, the signal processing circuit comprises an instruction RAM (random access memory). The signal processing circuit further comprises an external memory interface circuit for carrying out input/output control of the input signal in the signal processing operation.
In the meanwhile, the external memory must be initialized previous to the signal processing operation. In initialization of the external memory, the signal processing circuit writes zero data in all addresses of the external memory by using the external memory interface circuit. The signal processing circuit carries out the initialization of the external memory in accordance with an initialization program. The initialization program is also transferred from the external central processing unit to the signal processing circuit and is temporarily stored in the instruction RAM. When the initialization completes, the initialization program in the instruction RAM is replaced by the predetermined signal processing program.
As apparent from the above description, the signal processing circuit requires the initialization program in order to carry out the initialization. Furthermore, the initialization program must be replaced by the predetermined signal processing program on completion of the initialization. This means that the signal processing circuit requires an extra or unnecessary time duration before the signal processing circuit starts the signal processing operation. In other words, the signal processing circuit requires an increased initialization time duration for carrying out the initialization. Moreover, the load on the external central processing unit increases because the external central processing unit must carry out transfer operation of the initialization program.